Atmospheric-pressure plasmas have prominent technical significance because in contrast with low-pressure plasma or high-pressure plasma no reaction vessel is needed to ensure the maintenance of a pressure level differing from atmospheric pressure. Accordingly, depending on the principle of generation, these plasmas can be employed directly in the production line. The need for cost-intensive chambers for producing a partial vacuum as used in low-pressure plasma technology is eliminated.
Various forms of excitation are distinguished:
- AC (alternating current) excitation
- DC (direct current) and low-frequency excitation
- Excitation by means of radio waves
- Microwave excitation
Atmospheric-pressure plasmas that have attained any noteworthy industrial significance are those generated by DC excitation (electric arc), AC excitation (corona discharge, dielectric barrier discharge, piezoelectric direct discharge and plasma jets as well as 2.45 GHz microwave microplasma).
Operating principle of a DC plasma jetEdit
By means of a high-voltage discharge (5–15 kV, 10–100 kHz) a pulsed electric arc is generated. A process gas, usually oil-free compressed air flowing past this discharge section, is excited and converted to the plasma state. This plasma passes through a jet head to the surface of the material to be treated. The jet head determines the geometry of the beam, and is at earth potential to hold back potential-carrying parts of the plasma stream.
Operating principle of a microwave plasma jetEdit
A microwave system uses amplifiers that output up to 200 watts of power radio frequency (RF) power to produce the arc that generates plasma. Most solutions work at 2.45 GHz. A new technology provides ignition and highly efficient operation with the same electronic and couple network. This kind of atmospheric-pressure plasmas is different. The plasma is only top of the electrode. That is the reason the construction of a cannula jet was possible.
Manufacturers use plasma jets for, among other things, activating and cleaning plastic and metal surfaces to prepare them for adhesive bonding and painting. Sheet materials up to several meters wide can be treated today by aligning a number of jets in a row. Surface modification achieved by plasma jets is comparable to the effects obtained with low-pressure plasma.
Depending on the power of the jet, the plasma beam can be up to 40 mm long and attain a treatment width of 15 mm. Special rotary systems allow a treatment width per jet tool of up to 13 cm. Depending on the required treatment performance, the plasma source is moved at a spacing of 10–40 mm and at a speed of 5–400 m/min relative to the surface of the material being treated.
A key advantage of this system is it can be integrated in-line in existing production systems. In addition the activation achievable is distinctly higher than in potential-based pretreatment methods (corona discharge).
It is possible to coat varied surfaces with this technique. Anticorrosive layers and adhesion promoter layers can be applied to many metals without solvents, providing a much more environmentally friendly solution.
- Wolf, Rory A., Atmospheric Pressure Plasma for Surface Modification, Wiley, 2012
- Fazeli, M.; Florez, J.; Simão, R. (9 November 2018). "Improvement in adhesion of cellulose fibers to the thermoplastic starch matrix by plasma treatment modification". Composites Part B: Engineering. 163: 207–216. doi:10.1016/j.compositesb.2018.11.048.
- Heuermann, Holger; et al. (June 2012). Various applications and background of 10-200W 2.45GHz microplasmas. 60th International Microwave Symposium. Bibcode:2012imsd.conf59386H. doi:10.1109/MWSYM.2012.6259386.
- Noeske M., Degenhardt J., Strudhoff S., Lommattzsch U.: Plasma Jet Treatment of five Polymers at Atmospheric Pressure: Surface Modifications and the Relevance for Adhesion; International Journal of Adhesion and Adhesives; 24 (2) 2004, pp. 171–177
- Buske C., Förnsel P.: Vorrichtung zur Plasmabehandlung von Oberflächen (Device for the plasma treatment of surfaces); EP 0986939
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- Basics of microwave driven atmospheric plasma